This description relates to a method of manufacturing light emitting diodes.
Recently, GaN-based light emitting diodes have attracted attentions as blue and green light emitting diodes. InxGa1-xN used as an active layer is known as a material that has a wide energy band gap and thus can emit light over an entire range of visible light according to the composition of In.
Such a light emitting diode has a wide range of applications including a sign board, a display device, a device for a backlight, a bulb, and the like, and its range of applications is gradually expanded. Thus, it is of great importance to develop a high-grade light emitting diode.
FIG. 1 is a sectional view schematically showing the configuration of a conventional light emitting diode (LED). An N—GaN layer (101), an active layer (102) and a P—GaN layer (103) are sequentially laminated on a sapphire substrate (100). From the P—GaN layer (103) to the N—GaN layer (101) is mesa-etched. A transparent electrode (104) and a P-metal layer (105) are sequentially formed on the P—GaN layer (103). An N-metal layer (106) is formed on the mesa-etched N—GaN layer (101).
The diode thus constructed is bonded to a molding cup using an adhesive (108). The N-metal layer (106) is wire-bonded to a first lead frame (109a) that is connected to an external lead wire. The P-metal layer (105) is wire-bonded to a second lead frame 109b that is connected to another external lead wire.
Now, operation of the LED will be described. When a voltage is applied through N and P electrodes, electrons and holes flow from the N—GaN layer (101) and the P—GaN layer (103) into the active layer (102) where the electrons and holes are re-combined to emit light.
The active layer (102) emits light through the top, bottom and lateral surface portions thereof. Light emitted through the top portion of the active layer emerges to the outside through the P—GaN layer (103).
However, since the LED is fabricated on a sapphire substrate having low thermal conductivity, it is difficult to smoothly dissipate heat produced during the operation of the device operation, resulting in degradation of characteristics of the device.
In addition, as shown in FIG. 1, electrodes cannot be formed on the top and bottom of the device and thus should be formed on the top of the device. Thus, a portion of the active layer should be removed. Accordingly, there are problems in that a light emitting area is reduced, it is difficult to implement a high-luminance and high-grade LED, the number of chips obtained from one wafer is decreased, a manufacturing process is complicated, and bonding should be performed twice during assembly.
Furthermore, after processes for LED chips are completed on a wafer, lapping, polishing, scribing and breaking processes are carried out to divide the wafer into unit chips. At this time, if a sapphire substrate is used as a substrate, there is a problem of yield reduction due to stiffness of sapphire and mismatch of cleavage planes between sapphire and GaN.
FIGS. 2a to 2e are sectional views illustrating a method of manufacturing a conventional improved LED.
First, as shown in FIG. 2a, an undoped GaN layer (122), an N—GaN layer (123), an InxGa1-yN layer (124), and a P—GaN layer (125) are sequentially formed on the sapphire substrate (121) using an MOCVD process.
Here, the N—GaN layer (123), the InxGa1-yN layer (124), and the P—GaN layer (125) constitute a basic light emitting structure.
Then, sequentially formed on the P—GaN layer (125) are a transparent electrode (126), a reflective film (127), a solder-reaction inhibition layer (128), and a metal layer (129) made of any one selected from Ti/Au, Ni/Au, and Pt/Au.
Thereafter, a base substrate (130) through which a current can flow is prepared. The base substrate (130) has first and second ohmic contact metal layers (131, 132) formed respectively on the top and bottom thereof. A solder (133) for attachment of an LED chip is formed on the first ohmic contact metal layer (131).
Then, the metal layer (129) of the light emitting structure is bonded to the solder (133) of the base substrate (130), as shown in FIG. 2a. 
Subsequently, laser is irradiated on the sapphire substrate (121) to separate the sapphire substrate (121) from the undoped GaN layer (122) (FIG. 2b).
Thus, through the laser irradiation, the sapphire substrate (121) is entirely removed from the undoped GaN layer (122). The undoped GaN layer (122) remains as a layer damaged from the surface thereof to a certain thickness. (FIG. 2c).
Therefore, as shown in FIG. 2d, the undoped GaN layer (122) is etched through a dry-etching process until the N—GaN layer (123) is exposed. Then, an N-electrode pad 141 is formed on the N—GaN layer (123) (FIG. 2d).
At this time, in order to form a plurality of LEDs, a plurality of N-electrode pads. (141) each spaced apart from one another are formed on the N—GaN layer (123).
Finally, cutting processes such as scribing and breaking are performed between the N-electrode pads (141) from the N—GaN layer (123) to the second ohmic contact metal layer (132), thereby obtaining separate devices (150, 160) (FIG. 2e).
However, this conventional technique also has the following problems.
That is, since the sapphire substrate is used as a substrate for forming an epi-layer thereon, lattice mismatch between GaN of the epi-layer and sapphire degrades the quality of the epi-layer, causing a lowered light emitting efficiency, a reduced electrostatic damage (ESD) level, deteriorated reliability, and the like.
In addition, although a nitride semiconductor substrate has been studied as an alternative substrate in order to solve the problems with this sapphire substrate, there is no substantial success. Since this nitride semiconductor substrate is expensive, the use thereof as disposables leads to a problem of increased manufacturing costs.